1. Field of the Invention
This invention relates generally to the production of iron oxides from flows of iron contaminated water and, more particularly, to an apparatus and method for producing a commercially marketable iron oxide product and a substantially iron free effluent from iron contaminated water.
2. Description of the Prior Art
Approximately one billion pounds of iron oxides are produced and consumed annually in the world. Iron oxides are used as pigments and colorants, where their low cost and non-hazardous characteristics are valued. Iron oxides are also used as propellants in automobile airbags, as industrial catalysts and as feed stocks to the magnetite, ferrite and toner industries. The world demand for iron oxides has recently grown by several percent per year and is expected to continue to grow over the next several decades.
Iron oxides are currently obtained from three sources. Iron oxides are mined from natural deposits, produced synthetically by the chemical industry and produced as a by-product of steel making. Natural iron oxides generally have an iron oxide content of 25-75%, with the balance of the material being alumino-silicates and associated elements. The moderate iron oxide content, the presence of contaminants and the large particle sizes limit the value of natural iron oxides unless they are intensively processed.
Iron oxides produced synthetically in chemical batch reactors produce the purest form of iron oxides and, consequently, demand the highest prices. Several processes are used to produce synthetic iron oxides. Examples of such processes are the PENNIMAN and LAUX processes in which iron oxides are produced through the oxidation of scrap iron. Because the starting materials in synthetic processes are often contaminated with other metals, the synthetic iron oxides produced are commonly contaminated with heavy metals. This condition limits their use in cosmetics, animal feeds, airbags and as industrial catalysts.
Regeneration oxides produced by steel mills as a by-product of pickling liquor have a very high iron oxide content (95-99%) but chloride contamination limits the utility of these materials. Most regeneration oxides are used as feed stocks to magnetite, ferrite and toner production processes.
The value of an iron oxide product is determined by its particular physical and chemical characteristics. For pigmenting and colorant applications, the value of the iron oxide is determined by its tinting strength, which is a direct function of the iron content and the particle size. The highest quality iron oxide pigments have iron oxide contents of greater than 90% and mean particle sizes less than 0.6 .mu.m. Synthetic iron oxides generally meet these particle size requirements with moderate processing, while natural iron oxides have mean particle sizes on the order of 1-2 .mu.m. The use of iron oxides in cosmetics and animal feeds as propellants in airbags and as catalysts is strictly limited by the presence of excessive concentrations of heavy metals. Both natural iron oxides and iron oxides produced synthetically from scrap iron contain concentrations of copper and chromium that are generally unsuitable for these high value uses.
Iron contaminated coal mine drainage is a common consequence of mining coal. In the United States alone, more than one billion pounds per year of iron (as FeOOH) is discharged from active and abandoned mines. Millions of dollars per day are spent in the United States to treat the polluted drainage from active and abandoned mine sites. At hundreds of additional sites where responsible parties do not exist, the contaminated water flows untreated into receiving streams. An estimated 4,000 miles of streams and rivers are polluted by untreated mine drainage in northern Appalachia alone.
Currently, iron contaminated waters are treated by active and passive treatment systems. However, these conventional treatment systems consider the metal rich product of treatment as a waste product that must be expensively disposed of. In the active systems, alkaline chemicals (such as NaOH, CaO, CaO(OH).sub.2 or Na.sub.2 CO.sub.3) are added to the contaminated mine water to promote the formation and precipitation of metal hydroxide solids or "sludge". The resulting sludge, which commonly contains calcium sulfate, ferric hydroxide, ferrous hydroxide, aluminum hydroxide and manganese oxide is generally disposed of by burial, land filling or deep mine injection. Active treatment systems require continuous chemical additions and management of the metal hydroxide sludge. It is not unusual for treatment costs at a reclaimed medium-size mine site to be on the order of $100,000 per year. Because contaminated mine drainage generally persists for decades, a financial situation develops that strains even the most efficient mining operations. Further, the high cost of chemical systems also makes active treatment financially impractical at most abandoned mine sites.
Passive systems utilize natural chemical and biological processes to treat contaminated mine water. Passive treatment systems typically contain wetlands constructed with organic substrates and vegetated with hydrophilic species. The contaminated mine water flows into the wetlands where, under the influence of microbial, botanical and natural chemical processes, metal contaminants in the mine water precipitate to the bottom of the wetland as oxides, hydroxides, sulfides and carbonates to form a metal rich "sludge". Unlike active systems, passive systems can provide reliable water treatment with very little operation and maintenance requirements. Passive treatment however has limitations. For example, the land required for a typical passive system is generally 5-20 times greater than that for an active chemical system.
In both active and passive systems, the accumulation of the precipitate sludge decreases the effective volume of the system and degrades the water treatment capabilities. Therefore, the sludge deposits must eventually be removed and disposed of. In active systems, where metal precipitations are fast and the collection systems generally small, sludge management is a major treatment operation. Semi-annual management efforts (removal and disposal) are typical for chemical treatment operations. Passive systems have much higher sludge storage capacities and, therefore, the sludge management efforts are less frequent. Typically, passive systems are constructed with enough storage volume to allow the accumulation of 10-20 years of sludge in the wetland. Eventually, even with passive systems, the sludge will have to be removed to assure continued effective treatment of the contaminated mine water.
The potential utility of sludge produced by mine water treatment systems has been investigated. However, efforts to recover valuable metal products from mine drainage have always been stymied by the highly variable chemical composition of treatment sludges. Sludge from active treatment systems has shown potential as a soil additive or a cementing material. Even these uses, however, have proven impractical because of the variable chemical compositions of the sludges produced from differing chemical treatment systems.
The use of sludge from passive systems has been given less investigation than that from active systems. An early attempt to use the precipitate from coal mines is disclosed in U.S. Pat. No. 887,043 to Heckman. In the Heckman patent, "coal mine sulfur" discharging from a coal mine is precipitated by impounding the drainage in a crudely constructed reservoir. The precipitate that is collected is a yellow substance containing silica, sulfuric acid, iron and traces of lime and magnesia. The Heckman patent describes the use of "coal mine sulfur" for pigmenting purposes, following its calcination either alone or with sulfuric acid. The Heckman patent appears to describe the mineral Schwertmannite, a commonly occurring precipitate from acidic coal mine drainage having an iron oxide content of 60-70%. The Heckman precipitate, while containing iron oxide, differs substantially from iron oxides produced and consumed in the world today. Heckman's coal mine sulfur has negligible pigmentary value today because of its acidic, impure chemistry. Further, the Heckman patent does not describe any features of the mine water or the impoundment design that affect the quality of the precipitate, its rate of formation or its recovery.
Therefore, it is an object of the invention to provide a method and apparatus for producing a relatively high quality iron oxide product from iron contaminated water. It is a further object of the invention to provide a method and apparatus for the production of a commercial grade iron oxide product through the passive treatment of iron contaminated coal mine drainage in which the iron oxide product does not require extensive further treatment to be commercially marketable.